Multiple photon excitation dynamics

A typical problem of interest at Los Alamos is the solution of the infrared multiple photon excitation dynamics of sulfur hexafluoride. This very problem has been quite popular in the literature in the past few years. (7) The solution of this problem is modeled by a molecular Hamiltonian which explicitly treats the asymmetric stretch ladder of the molecule coupled implicitly to the other molecular degrees of freedom. (See Fig. 12.) We consider the the first seven vibrational states of the mode of SF (6v ) the octahedral symmetry of the SF molecule makes these vibrational levels degenerate, and coupling between vibrational and rotational motion splits these degeneracies slightly. Furthermore, there is a rotational manifold of states associated with each vibrational level. Even to describe the zeroth-order level states of this molecule is itself a fairly complicated problem. Now if we were to include collisions in our model of multiple photon excitation of SF, e wou d have to solve a matrix Bloch equation with a minimum of 84 x 84 elements. Clearly such a problem is beyond our current abilities, so in fact we neglect collisional effects in order to stay with a Schrodinger picture of the excitation dynamics. 

Figure 12. Schematic of multiple photon excitation dynamics of SFe. Groups of levels show lowest three vs vibrational states. Higher states are split by rotational interactions with vibrational motion.

Laser-induced reaction has been widely used to stimulate gas-surface interaction. Lasers are also used to probe molecular dynamics in heterogeneous systems as well. In the applied area, the laser photochemical techniques are successfully applied to produce well defined microstructures and new materials for microelectronic devices (1). Enhanced adsorption and chemical reaction on surfaces can be achieved by a photoexcitation of gaseous molecules, adsorbed species as well as solid substrates. The modes of the excitation include vibrational and electronic states of the gaseous species and of the adsorbates surface complexes. Both a single and a multiple photon absorption may be involved in the excitation process. 

Note that in the case of fluorescence, where the energies involved indicates that spontaneous emission in the form of a simple single transition should dominate. Nevertheless the typical pathways back to the ground state appear to involve multiple transitions where the excited state interchanges low energy photons with the environment. We thus have a case where the dynamics of the environment may facihtate a more efficient but complex multistep pathway back the ground state than spontaneous emission provides. 

Initial femtochemistry theoretical models were simply generalizations of those for the single-photon DIET processes, i.e., as dynamics induced by multiple electronic transitions (DIMET) [100]. The idea is simply that even if the excited state residence is too short to cause excitation to a ground state continuum after resonant scattering (tR < tc), it can still cause some vibrational excitation in the ground state. If resonant... 

The experimental setup for the broadband CARS is rather simple because only two pulses are needed for three-color CARS emission, as shown in Fig. 5.4a a broadband first pulse impulsively promotes molecules to vibrationally excited states through a two-photon Raman process, and a delayed narrowband second pulse induces anti-Stokes Raman emission from coherent superpositions to the ground state [29]. By changing the delay time for the second pulse, therefore, one can expect to probe dynamical behaviors of multiple RS-active modes. Such a two-dimensional observation in the time-frequency domains should be effective for detailed analysis of nanomaterials. 

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